1. Field of the Invention
The present invention relates to an amidite for nucleic acid synthesis suitable for synthesizing a nucleic acid, and a nucleic acid synthesizing method using the amidite.
2. Description of the Related Art
Solid-phase synthesis of nucleic acids was initiated as long ago as more than 20 years, and automatic synthesizers were already sold then. Solid-phase synthesis of a nucleic acid is carried out, for example by making a nucleic acid material (amidite) combine in a condensation reaction with a solid carrier (e.g. CPG) in which a nucleoside is immobilized; this condensation reaction needs to take place, with only a phosphoric acid portion in the amidite and a hydroxyl group in another amidite being involved in the condensation reaction, and other reactive groups not being involved in the condensation reaction. Therefore, it is necessary to prevent an exocyclic amino group, etc. contained in a base of an amidite used from being involved in the condensation reaction by introducing a protective group, and to eliminate (remove) the protective group after the condensation reaction has finished completely. Conventionally, benzoyl group, isobutyryl group and the like have been used for protective groups introduced into exocyclic amino groups in bases, and a method of allowing concentrated ammonia water to act at 55° C. for 8 hr to 15 hr so as to remove these protective groups has been common.
However, for example, in order to modify functional artificial nucleic acids such as primers, probes, antisense DNAs, siRNAs, etc. with fluorescent labels or the like, improved amidites which make it possible to remove a protective group and obtain a nucleic acid under more moderate conditions are hoped for. For example, in the related art, amidites for nucleic acid synthesis represented by Structural Formulae (4) to (6) in FIG. 14 have been reported as nucleic acid amidites which enable protective groups therein to be removed by diazabicycloundecene (DBU) that is a bulky base (refer to Acta Chem, Scand., B37, 263 (1983) and J. Org. Chem., 54, 1657 (1989)). However, since the amidites for nucleic acid synthesis represented by Structural Formulae (4) to (6) are unstable in acetonitrile that is an aprotic solvent (refer to Tetrahedron Letters 46, 6729 (1990)), they are not suitable for practical use. Additionally, although it has also been reported that amidites for nucleic acid synthesis represented by Structural Formulae (7) to (9) in FIG. 15 enable protective groups therein to be removed under moderate conditions (in pyridine, 0.5M, DBU, 16 hr) (refer to Tetrahedron 40, 4171 (1992) and Nucleodied & Nuclrotides 13, 2059 (1994)), they are problematic in that nucleic acid bases are alkylated owing to highly-concentrated DBU and deprotection for a long period of time. In addition, although it has also been reported that amidites for nucleic acid synthesis represented by Structural Formulae (10) to (12) in FIG. 16 enable protective groups therein to be removed under moderate conditions (in methanol, K2CO3) (refer to Tetrahedron Letters 46, 6729 (1990) and Nucleic Acids Research 21, 3493 (1993)), they are problematic in that esters, etc. decompose because K2CO3 that is a base is used in methanol that is a protic solvent.
Thus, as things stand at present, development of an amidite for nucleic acid synthesis which enables a protective group therein to be removed under moderate conditions and can be practically used, and of a nucleic acid synthesizing method using the amidite for nucleic acid synthesis is still hoped for.